Replace most uses of Boost.{Filesystem,Process}

With the `ghc::filesystem` dependency from the previous commit. If we
can replace the rest of the Boost.Filesystem dependency then we can get
rid the one nasty runtime dependency we have, and it will make
implementing the chainloading simpler since can reuse more code without
bringing in Boost.

src/common/utils.cpp

@@ -16,12 +16,12 @@
 
 #include "utils.h"
 
+#include <stdlib.h>
+
 #include <sched.h>
 #include <xmmintrin.h>
-#include <boost/process/environment.hpp>
 
-namespace bp = boost::process;
-namespace fs = boost::filesystem;
+namespace fs = ghc::filesystem;
 
 using namespace std::literals::string_view_literals;
 
@@ -40,13 +40,12 @@ constexpr char disable_watchdog_timer_env_var[] = "YABRIDGE_NO_WATCHDOG";
 constexpr char temp_dir_override_env_var[] = "YABRIDGE_TEMP_DIR";
 
 fs::path get_temporary_directory() {
-    const bp::environment env = boost::this_process::environment();
-    if (const auto directory = env.find(temp_dir_override_env_var);
-        directory != env.end()) {
-        return directory->to_string();
-    } else if (const auto directory = env.find("XDG_RUNTIME_DIR");
-               directory != env.end()) {
-        return directory->to_string();
+    // NOLINTNEXTLINE(concurrency-mt-unsafe)
+    if (const auto directory = getenv(temp_dir_override_env_var)) {
+        return fs::path(directory);
+        // NOLINTNEXTLINE(concurrency-mt-unsafe)
+    } else if (const auto directory = getenv("XDG_RUNTIME_DIR")) {
+        return fs::path(directory);
     } else {
         return fs::temp_directory_path();
     }
@@ -105,13 +104,13 @@ bool pid_running(pid_t pid) {
     // processes and zombies, and a terminated group host process will always be
     // left as a zombie process. If the process is active, then
     // `/proc/<pid>/{cwd,exe,root}` will be valid symlinks.
-    boost::system::error_code err;
+    std::error_code err;
     fs::canonical("/proc/" + std::to_string(pid) + "/exe", err);
 
     // NOTE: We can get a `EACCES` here if we don't have permissions to read
     //       this process's memory. This does mean that the process is still
     //       running.
-    return !err.failed() || err.value() == EACCES;
+    return !err || err.value() == EACCES;
 }
 
 std::string url_encode_path(std::string path) {